The invention relates to a device for reducing amplitude noise of a light radiation as well as to a method for reducing amplitude noise of a light radiation.
Optical noise reducers, also known to a person skilled in the art under the English technical term “noise eater”, are known from prior art in various embodiments and are being used in various ways in optical technology.
Laser radiation is generally full of undesirable amplitude noise (also referred to as intensity noise or power noise), wherein the noise can extend over a wide frequency range of below 1 Hz up into the GHz range.
A noise reducer known from prior art represents a combination of a laser beam modulator, a light detector and an electronic controller, said controller holding constant the light intensity at the exit of the arrangement by compensating for the changes of intensity registered by the light detector by means of an opposed modulation of the light wave. A photodiode is generally used as light detector because even intensity fluctuations of very high frequencies can be registered with this. An electro-optical modulator or an acousto-optical modulator is generally used as laser beam modulator.
With an electro-optical modulator, also referred to as Pockets cell, the amplitude of a laser beam can be altered, that is modulated, almost without delay with the help of an electric signal. For this purpose, the beam of light to be modulated, which is polarized linearly or has been polarized with the help of a polarizer, runs through one or multiple non-linear crystals which have a linear electro-optical effect. A beam of light running through a non-linear crystal is split into an ordinary partial beam and an extraordinary partial beam. The crystals are furnished with electrodes so that an electrical field can be produced inside of the crystals by applying an electric voltage. Due to the linear electro-optical effect the refraction indices no for the ordinary partial beam and ne for the extraordinary partial beam change.
Upon exiting a crystal arrangement the partial beams reunite to form one beam of light having an altered polarization state compared to the incident beam. Thereafter, the beam regularly runs through a so-called analyzer, that is to say a polarizer, which suppresses a polarization component.
By altering the applied electrical field the polarization state of the light wave can in front of the analyzer be changed continuously between unaltered polarization, circular polarization and linear polarization rotated by 90 degrees. The beam exiting the analyzer is again linearly polarized with an amplitude that is altered compared to the incident beam.
The voltage change needed to change the amplitude from minimal to maximum value is referred to as half-wave voltage Uλ/2. The transmittance T of the modulator as function of the voltage applied U is specified roughly by the function of the subsequent equation I:
                              T          ⁡                      (            U            )                          =                              cos            2                    ⁡                      (                                          π                ⁡                                  (                                      U                    -                                          U                      0                                                        )                                                            2                ⁢                                  U                                      λ                    /                    2                                                                        )                                              (        I        )            
Here, the offset voltage U0 is the voltage needed for maximum transmittance. The transmittance of the modulator can hence by variation of the applied voltage between U0 and Umax=U0+Uλ/2 theoretically be varied between 0% and 100%.
In a noise reducer the amplitude of the transmitted beam of light is held constant with the help of a controller, in that the voltage applied to the modulator is varied correspondingly. Depending on the setting of the set value, the voltage applied to the modulator then varies around an operating point Us.
In the longitudinal electro-optical modulation the electrical field is aligned parallel to the direction of propagation of the light. In this arrangement it is viewed as advantageous that the occurrence of natural birefringence, causing a strong temperature dependence of the modulator, can be avoided. In contrast, the transversal electro-optical modulation offers the advantage that the half-wave voltage needed is lower when a suitable crystal length is chosen. But then there is in this case a generally undesirable natural birefringence, that is, the crystal alters the polarization state of the beam of light even without an electrical field being applied.
Since the refraction indices no and ne of the crystals possess different temperature coefficients, the polarization state of the light and consequently the amplitude of the beam of light exiting the polarizer become temperature dependent. In order to avoid this, usually pairs of crystals of the exact same length are used, arranged one after the other, the optical axes of which are rotated by 90 degrees against each other, so that the natural birefringence of the first crystal is compensated for by that of the second crystal. Thereby, the temperature dependence of the modulator can largely be avoided. Typically, crystals are manufactured with a length tolerance of about 0.1 mm. Due to the selection of two crystals well synchronized with respect to their length, often crystals are employed which differ in length only by some 10 μm. Within the scope of the present invention this is understood to be “of the same length”.
In the semiconductor industry noise reducers are often employed to reduce the noise of laser radiation in inspection systems. Increasingly higher demands are nowadays placed on the life-span of all of the components of such inspection systems, 20 000 to 40 000 hours in continuous operation (24 hours per day at 7 days a week) are common. This requirement can hardly be fulfilled by the hitherto conventional noise reducers having an electro-optical modulator. As it is known from the literature (see for example M. N. Satyanarayan and H. L. Bhat: “Electrical and Optical Characterisation of Electrochromic Damages in KTP Single Crystals”, Journal of the Korean Physical Society, 32 (1998), pages p 420-p 423), birefringent crystals which are permanently exposed to an electrical DC field show an optical transparency declining over time. The effect, also visually perceptible as gray discoloration, is referred to as “electro-chromatism”. It occurs especially strongly in materials having a high ion conductivity, which is why an impairment of the crystal lattice by migration of ions seems likely as cause.
A material having high ion conductivity is for example potassium titanyl phosphate (KTiOPO4, KTP). Even with lower electrical field strengths, as they are common in electro-optical modulators, within minutes this crystal shows gray discolorations, which make it entirely unusable. In contrast, the effect does not occur when merely alternating fields are applied to the crystal. Hence, this crystal is not suitable for application in a standard noise reducer in which in addition to an AC voltage portion a resulting DC voltage portion and consequently a resulting DC field are also always present.
Other crystals have considerably lower ion conductivities and can hence be used in a noise reducer over a longer time period depending on the used field strength. With the use of especially long crystals and accordingly lower field strengths needed the life-span can be increased, however, the maximum lengths are restricted by the manufacturing processes available today. Hence, even with these crystals the life-span required in the semiconductor industry cannot be ensured.